620 
SCANDINAVIAN FISHES. 
as broad and expanded like a wing. But the branchio- 
stegal membranes are entirely hidden under the common 
dermal covering, so that the Artedian character of the 
order holds good as an external one. The pelvic bones 
are generally wanting, but sometimes present, as in the 
Balistoids, in which case they are united into a large, 
scythe-shaped bone, inferiorly bounding and supporting 
the abdominal cavity, even when the pectoral tins have 
disappeared or have been reduced to a spiny, osseous 
projection at the hind extremity of this bone. The 
remainder of the skeleton is marked not only by its 
weak ossification, but also by the small number of the 
vertebrae, which may sometimes (in the Coffer-fishes) 
sink to 14 and never exceeds 30. Sometimes, as in the 
Tetrodonts, the upper spinous processes (neural spines) 
of the first 5 or 6 vertebrae are longitudinally cleft in 
two, with the halves laterally extended. Ribs are want- 
ing in most cases, but present in the Balistoids. As 
the caudal fin is frequently of extraordinary size — though 
with few, less than 13, branched rays — , its insertion 
in the skeleton has also undergone proportionate increase 
in strength and firmness, partly by means of large 
hypural bones, which are firmly united in the Coffer- 
fishes into a compressed tip, hard as ivory, partly by 
high and longitudinally extended, neural and haemal 
spines on the last caudal vertebrae. Sometimes however, 
as in the Sunfishes, the caudal fin is a secondary growth 
and finds an extremely feeble support in the more or 
less atrophied tip of the tail. 
In spite of the considerable differences that occur 
both in the form and covering and also in the internal 
structure, the Plectognates together form a natural group, 
with persistent traces of the Ganoid type, which give 
them an appearance unusual in the fauna of our day. 
The nearest approach to the preceding forms we find, 
as Cope has already pointed out", in a comparison be- 
tween the Balistoids (especially the genus Triacanthus, 
with its free maxillaries) and the Leather-fishes ( Acan - 
thuridce, a family within the comprehensive series of 
Mackerel-fishes), with the same form of body, the same 
elongation and deepening of the forepart of the head 
and shortening of the jawbones, the same union of the 
posttemporal and mastoid bones, and the same length 
of the pelvic bones, which in the Acanthuroids, however, 
are not confluent. Set side by side with the skeleton of 
an Acanthurus the skeleton of a Batistes seems at the 
first glance to claim a position within the same family. 
The systematic separation between the Plectognates and 
the other Physoclysts is thus by no means too great to 
admit of their being ranged side by side with each other. 
About 200 species of Plectognates are known. The 
greatest wealth of forms belongs to the tropical seas; 
but one or two species penetrate into rivers, or even 
pass their whole life there. To man these fishes are of 
only slight value. Their flesh is in general not eaten * 6 , 
and many of them are even poisonous. They attract 
attention, however, both by their extraordinary form 
and often by their beauty of colour, as well as by their 
singular manner of life. Most of them are shore-fishes, 
but some belong to the surface-regions of the ocean; 
they do not include any deep-sea forms of pronounced 
character, although two species have been met with 
below the 100-fathoms lineb Most of them are ren- 
dered incapable of any vigorous motion by their hard 
and stiff dermal covering or by their clumsy form. 
By the small size of the gape most of them are com- 
pelled to find subsistence in small animals or in pieces 
torn from larger ones. Their food consists chiefly of 
corallines, jelly-fishes, worms, or mollusks. The lively 
colours that adorn many of them are reminiscences of 
their life in the coral-groves, where they associate with 
scaly-finned Chsetodonts of no less varied hues, or among 
the seaweeds with their shifting colours. The species of 
plainer dress resemble the sand in colour, or are attired 
for a life in the open sea. Their powerful teeth may 
well serve to protect them from attack, but as a rule 
they have quite other weapons of defence. Many of 
them have been famed from ancient times for their 
faculty of filling the oesophagus, which is capable of 
extraordinary distension, with air. In this manner the 
body is expanded, sometimes to a perfect globe, and 
the dermal spines are erected in all directions into a 
front of pointed spikes. “One day,” wrote Darwin from 
Bahia in 1832 d , “I was amused by watching the habits 
" Trans. Amer. Philos. Soc., Philad., vol. XIV, n. set., p. 458. 
6 According to Schleqel (Sieb., Fn. Japan.'), however, several of these fishes are eaten by the Japanese: one Tetrodon is even stated 
to compose the principal food of the poorer classes during winter, while the fisherman is forbidden, under pain of heavy penalties, to expose 
for sale other species of the same genus. 
c Both these finds, however (see Gunther, Deep /Sea Fish. Chall. Exped., p. 266), may have entered the trawl while it was being 
sunk or hauled in. 
d A Naturalist' s Voyage (London, ed. 1884) p. 13. 
